1. Field of the Invention
The present invention is a treatment process for hexavalent chromium {Cr(VI)}-bearing soils, sediments, industrial wastes, fill, or other materials that have become contaminated, have been released into the environment, or have been generated in the process of producing chromium compounds. Specifically, a process is disclosed which reduces the toxic hexavalent form of chromium, Cr(VI), to trivalent chromium, {Cr(III)}, through the addition of ascorbic acid (CAS: 5081-7; C.sub.6 H.sub.8 O.sub.6).sup.1 to Cr(VI)-bearing soils/materials. The ascorbic acid is mixed with the Cr(VI)-bearing material in situ (in place) without the need to remove the soils/materials from the location in which they repose or can be applied ex situ (above the ground surface) to Cr(VI)-bearing material that has been excavated or otherwise removed from the ground or other repository. The process may be practiced at ambient temperature and atmospheric pressure. The reduction of Cr(VI) to Cr(III) is rapid and does not require the addition of other agents or prior alteration of the pH of the Cr(VI)-bearing soils/materials. The process is particularly applicable to soils exposed to Cr(VI) as a result of either 1) chromate chemical releases, or 2) mixing with Cr(VI)-bearing residue that was generated during the production of chromium compounds such as those derived from the processing of chromite ore, as described by Austin.sup.2 and Westbrook,.sup.3 or to chromite ore processing residue containing undesirably high levels of Cr(VI).
2. Regulatory and Factual Background
Pursuant to efforts to remediate hazardous waste sites across the United States, chromium, chromate, and/or Cr(VI) have been identified as contaminants of concern at a significant number of National Priority List (Superfund) sites under the Comprehensive Environmental Responsibility and Liability Compensation Act.sup.4 as well as at many hundreds of other sites where chromium chemicals have been released into the environment. Cr(VI) is categorized as a human inhalation carcinogen,.sup.5 and many forms of Cr(VI) are highly soluble and mobile in the environment. Inhalation of Cr(VI)-bearing airborne particulates is the most significant pathway of potential exposure to humans. The highly soluble and mobile characteristics of many chromate compounds are further concerns for Cr(VI)-bearing soils/materials as Cr(VI) contamination can spread significant distances from a source location via surface water runoff or groundwater migration. There is also concern about ingestion of water containing high total chromium concentrations which is reflected in the federal drinking water chromium concentration standard of 0.1 mg/L.
In contrast to Cr(VI), Cr(III) is not categorized as a human carcinogen and is even considered an essential trace nutrient for mammals. Although some soluble forms of Cr(III) have been shown to be toxic to certain aquatic species, soluble Cr(III) is rarely encountered in aquatic systems. Trivalent forms of chromium are most frequently encountered in insoluble forms in the environment..sup.6 Thus, Cr(III)-bearing soils/materials are considered a significantly lesser health concern than are Cr(VI)-bearing materials. Cr(VI) is most frequently the focus of remediation decision-making at sites where elevated levels of chromium have been identified in soils.
Under the Resource Conservation and Recovery Act (RCRA), the U.S. Environmental Protection Agency (USEPA) has established testing criteria for determining when a waste or soil containing chromium is considered to be a hazardous waste. When the USEPA standardized Toxicity Characteristic Leaching Procedure (TCLP) is applied to a material and the total chromium concentration present in the leachate is greater than 5 mg/L, the material is designated a "characteristic hazardous waste," subject to the RCRA treatment, storage, and disposal regulations.
Cr(VI) is not naturally found in most soil-water environments. Thus, essentially all Cr(VI) contamination encountered in soils, sediments, wastes, and other materials is the result of human activities..sup.7 Much of the Cr(VI) contamination is the result of spilled or discarded Cr(VI)-bearing materials, including such widely used chromate chemicals as K.sub.2 Cr.sub.2 O.sub.7, Na.sub.2 CrO.sub.4, and CrO.sub.3..sup.8 Such Cr(VI) contamination can cause physical, chemical, and/or biological changes that alter the properties of the contaminated soils/materials, including pH, permeability, porosity, salinity, oxidation-reduction (redox) potential, and microbial population.
Another type of Cr(VI) contamination of soils is associated with chromite ore processing residue (COPR). This residue has unique physical/chemical properties (e.g. ranging in particle size from silt and sand grain-size spherical particles to agglomerated cement-like monoliths larger than baseballs), as described by James,.sup.9,10 compared to most soils in their natural state. This residue material is produced by the roasting of chromite ore in a kiln under alkaline oxidizing conditions to commercially extract and produce various chromium compounds..sup.3,11 Because this extraction process is incomplete, the COPR contains residual soluble and insoluble Cr(VI)..sup.12 The COPR is also quite alkaline, typically exhibiting a pH greater than 11, due to the use of lime (CaO) and soda ash (Na.sub.2 CO.sub.3) in the roasting process..sup.3 Its color can vary based on the ore source and the materials used in the roasting process, although grayish-black and reddish-brown materials are typical.
For many decades of the twentieth century until the 1970s, COPR was used with other fill material to reclaim marshy areas near the chemical manufacturing facilities that produced it. The Cr(VI) and total chromium content of these COPR-bearing soils/materials vary widely based on many factors, including the source and characteristics of the chromite ore processed, the materials added while processing the ore, the actual processing conditions at the time the residue exited the process, the nature of the soils or fill material with which the COPR was mixed during its deposition, and the degree of weathering that has occurred since placement in the environment.
For soils that are highly-enriched with COPR, the total chromium concentration can exceed 30,000 mg/kg, with about 33% to 66% existing as Cr(VI). However, such elevated concentrations of total chromium and Cr(VI) in COPR-bearing soils are rarely encountered except at sites that were filled predominantly with COPR. Typical COPR-bearing soils contain total chromium concentrations from several hundred to 4,000 mg/kg with Cr(VI) concentrations representing about 1% to 8% of the total chromium. The other major cations in COPR-enriched soils are typically iron, aluminum, calcium, and magnesium, the concentrations of which also may vary widely according to the chromite ore source, processing conditions, and the mixture of COPR with other fill material.
Compared to COPR-bearing soils, natural soils in the United States contain from 1 mg/kg to 2,000 mg/kg of total chromium with a mean of 54 mg/kg.sup.14 and negligible concentrations of the chromium existing as Cr(VI). Also, in COPR-bearing soils that have been mixed with sediments or organic-rich soils, the Cr(VI) concentrations may approach non-detectable levels (.ltoreq.5 mg/kg) due to the reducing conditions of the soil matrix,.sup.12,15 even though the total chromium concentration may be greater than 10,000 mg/kg.